1. Field of the Invention
The invention relates to an apparatus for defoaming blood, having a surface which comes into contact with blood and which is coated with a defoamer, and to such a defoamer.
The invention furthermore relates to a use of a surface coming into contact with blood for defoaming blood in an extracorporeal circulation.
2. Related Prior Art
An apparatus of the abovementioned type is disclosed in EP 0 774 285.
Such apparatuses are used for defoaming blood, for example during heart surgery. In open-heart surgery, a heart-lung machine takes over the functions of the patient's heart and lung in an extracorporeal circulation with the aid of pumps and oxygenators. During this, blood, in particular blood aspirated from a surgical wound, is often mixed with air with the formation of foam. This foam or foam bubbles must be removed from the blood before it is reintroduced into the patient who otherwise is exposed to the risk of embolisms.
Thus, for example, an extracorporeal system comprising an oxygenator, blood reservoirs and cardiotomy reservoirs and also blood tubing and filters etc. is expected not only to supply oxygen to and remove CO2 from the blood but also to remove completely the foam caused by the formation of bubbles. Blood filters and other apparatuses such as, for example, bubble traps, separating cyclones and the like, which are integrated into an extracorporeal circulation, cannot replace the use of defoamers in the blood reservoir and especially in the cardiotomy reservoir.
Defoaming is normally achieved by contacting the blood with a very large hydrophobic surface which is coated with a defoaming agent. In particular, open-celled polyurethane foams or textile materials, for example polyester materials, are used as a surface here.
The surfaces are coated with compounds which form a continuous film at the liquid/gas interface and thereby make it possible for the liquid to be degassed to form the smallest surface area. This “defoamer” destroys the gas bubbles.
The by far most frequently used defoamer for coating surfaces coming into contact with blood is silicone oil (polydimethyl-siloxane) or a mixture of polydimethyl-siloxane and silicone dioxide which is marketed by Dow Corning under the trade names “Simethicon®” and “Antifoam A®”.
These defoamers have the disadvantage of being slowly washed off from the surface by the streaming blood, thus reducing their effectiveness. As a result, after a while the blood is no longer defoamed sufficiently. The gas bubbles which have not been removed from the blood may cause embolisms in the patient. Moreover, the washing out means that during protracted surgical procedures silicone oil and silica particles may enter the patient and likewise cause embolisms there. This risk is furthermore increased by the fact that presently materials which mainly have improved hemocompatibility are used for the extracorporeal circulation. These then have, for example due to heparinization, a hydrophilic surface so that washed-off silicone oil cannot be trapped anymore by hydrophobic interactions of hydrophobic surfaces in the extracorporeal circulation, before entering the patient.
EP 0 774 285, mentioned at the outset, discloses coating of a hydrophobic surface with a defoamer which contains a triglyceride with at least one fatty acid comprising from 14 to 24 carbon atoms. It is preferred to use castor oil here. The defoamer may also contain a hydrophobic component, for example a silicone compound.
However, this defoamer proved to be much less effective than, for example, Simethicon®.
Moreover, it turned out that in the case of a castor oil-containing defoamer, too, as in the case of surfaces coated only with silicone, the oil is, after protracted use of said defoamer, washed off by the streaming blood, enters the patient and may cause a fat embolism there.
WO 95/28184 discloses a coating of polymers or metals, which has triblock copolymers of polylactone-polysiloxane-polylactone, the siloxane being dimethylsiloxane and the lactone being caprolactone. This coating proved to be particularly biocompatible and suitable for use in extracorporeal circulations, since, in this case, the blood comes into contact with a particularly large surface. Said coating may be applied in a stable manner not only to porous membranes but also to metal surfaces, whereby they became biocompatible.
However, the coating with triblock copolymers proved ineffective when used as defoamer.